Add NULL pointer check on NSError and remove unused code.

Hi,

These two comments are in reference to Issue #1652.

Not checking for a NULL value in `*error` was causing an EXC_BAD_ACCESS exception when the block in the following selector returned NO: 

`+ (instancetype)expectationWithSourceKeyPath:(NSString *)sourceKeyPath destinationKeyPath:(NSString *)destinationKeyPath evaluationBlock:(RKMappingTestExpectationEvaluationBlock)evaluationBlock`

`if (error) *error = [self errorForExpectation:expectation
                                                     withCode:RKMappingTestEvaluationBlockError
                                                       userInfo:userInfo
                                                   description:description
                                                         reason:reason];`

The second commit was to remove the assignment to `*error` on line 239; its immediately overwritten on line 254 so it seems pointless:

                   `NSMutableDictionary *mutableUserInfo = [userInfo mutableCopy];
                     [mutableUserInfo setValue:blockError forKey:NSUnderlyingErrorKey];
                     NSString *reason = [NSString stringWithFormat:@"expected to %@ with value %@ '%@', but it did not",
                                         expectation, [event.value class], event.value];
                     *error = [self errorForExpectation:expectation
                                               withCode:RKMappingTestEvaluationBlockError
                                               userInfo:mutableUserInfo
                                            description:[blockError localizedDescription]
                                                 reason:reason];
                     
                      *error = blockError;`